Nissan Quest Tire Cupping/Replacements

My 97 Quest tires showed cupping at 40K. Tires were rotated every 7500 miles per manual. Dealer's explanation was due to more weight in front, tires in back would tend to bounce more, causing the cupping. Does this make sense?Original tires are General AMEMRI G48 size P205/75R15. Any suggestions on a replacement tire?